1. Field of the Invention
This invention involves an improved evaporator arrangement for a home refrigerator.
2. Prior Art
FIG. 1 shows a conventional frost-free refrigerator using a single evaporator 10. A fan 12 moves air across the evaporator 10 while the compressor (not shown) is running, which cools the air. Most of the cold air goes into the freezer compartment 14. A small portion of the cold air is used to cool the fresh-food compartment 16. An electric heater 18 is energized with the evaporator fan 12 and compressor off to defrost the evaporator coil. This arrangement is used in virtually all U.S. refrigerators with automatic defrost.
The chief advantage of the arrangement shown in FIG. 1 is simplicity and low cost due to use of only one evaporator and one fan. The single evaporator coil also reduces the space requirement, as compared to two evaporator systems.
The chief disadvantage with the conventional arrangement shown in FIG. 1 is the high energy consumption associated with using a refrigerant at a single evaporating temperature to cool both compartments. The refrigerant temperature needs to be below the freezer temperature, while an efficient system could cool the 5 fresh-food compartment using evaporator temperatures that are 30.degree. to 40.degree. F. higher than those required for the freezer. Since roughly half of the cooling load comes from the fresh-food compartment, the potential energy savings amount to 20% or more for a system that efficiently uses two evaporating temperatures.
There have been several different types of refrigerators that use two evaporators. The "brute-force" solution is to use two completely independent circuits with two compressors. This approach adds a large cost penalty for the additional components. In addition, the theoretical energy savings can be negated by the lower efficiency associated with using two smaller compressors instead of one larger compressor, because compressor efficiency generally worsens at small capacities.
The Lorenz cycle is another approach that uses two evaporators. It uses two evaporators connected in series at essentially the same evaporating pressure. Two evaporating temperatures are achieved using a zeotropic blend of two or more refrigerants as the working fluid combined with internal heat exchangers. The evaporating temperature of a blend increases as the more volatile component evaporates and the liquid becomes richer in the less-volatile component. An internal heat exchanger is used so that two evaporating temperatures are created. Testing has shown that this arrangement gives energy savings approaching 20% with hydrocarbons or HCFCs (hydrochlorofluorocarbons). A major problem has been inability to find a suitable nonflammable, chlorine-free refrigerant blend. Getting the proper refrigerant charge for each component in a blend is also a problem which requires solution.
Other refrigerators use a solenoid valve to switch between two evaporators. A typical arrangement continuously cools the freezer evaporator and uses the solenoid valve to allow refrigerant into the second evaporator only when required to cool the fresh-food compartment. This arrangement is common in Asian refrigerators, and is used to achieve independent temperature control for each compartment. It usually does not provide significant energy savings since the refrigerant temperature is still below the freezer temperature when cooling the fresh-food compartment.
The tandem refrigeration system as disclosed in U.S. Pat. No. 5,406,805 is a recent improvement to the two-evaporator configuration. This prior art system uses two forced-convection evaporators, one for each compartment and each having its own dedicated fan. The control only runs one evaporator fan at a time. When the compressor first comes on, only the fresh-food evaporator fan runs. Once the fresh-food compartment is cooled, the controls turn the fresh-food fan off and then turn on the freezer fan. Defrost is achieved by running only the fresh-food fan and activating an optional solenoid valve to allow free circulation of refrigerant between the two evaporators. A thermosyphon effect allows heat from the fresh-food compartment to defrost the freezer evaporator without the need of an electric heater. This defrost method requires that the fresh-food evaporator be physically lower than the freezer evaporator to allow natural convection to work. Tests have demonstrated energy savings of 10 to 20 percent compared to conventional single-evaporator systems. While the tandem system is a major improvement compared to conventional single-evaporator systems, it still requires two evaporators and two evaporator fans.